The present invention relates to a process for patterning an oxide transparent electrically conductive film and, more specifically, a process for micro-patterning an Indium-Tin-Oxide (ITO) film. Also, the present invention relates to a process for patterning a transparent electrode to form a high density flat panel display.
Efforts have been made to further advance a technology for providing a high density and high image quality flat panel display. A current high image quality flat panel display is realized in the form of an active matrix type liquid crystal display. The active matrix type display includes a transparent electrically conductive film arranged mosaically in one substrate to be used as a pixel electrode and a switching element formed for each pixel, and controls each pixel electrode independently. Such a structure is made by applying a technology, of an integrated circuit or microelectronics.
As a transparent electrically conductive film for forming a pixel electrode, an ITO film is most generally used. With a higher pixel density, a size of one pixel is micronized more, and a pixel size of approximately 40 xcexcm in square is required. In the meantime, a screen has been enlarged, which resulted in the increase in the number of pixels. In the case of a QSXGA display, approximately 15 million pixels must be formed. Moreover, even if one pixel is defective, its substrate becomes a defective product. Thus, a more precise and accurate patterning technology of an ITO film is required.
A chemically amplified resist was developed for a microfabrication technology used to form a submicron order feature of VLSI. The chemically amplified resist overcomes an optical resolution limit of a conventional resist, and shows high resolution and high sensitivity to a deep ultraviolet light. Generally, the chemically amplified resist contains a photo-acid generating material such as onium salt, which is photolyzed to generate an acid and the acid reacts on an acid labile group, thereby changing solubility of the resist. At this time, the acid generated by the photolysis serves as a catalyst and takes part in reaction by a number of times, and thus high photosensitivity of the resist is provided.
This chemically amplified resist is also useful for a micro-patterning of an ITO film, which is carried out to form a transparent electrode as a pixel electrode of a high density flat panel display. Particularly, a negative chemically amplified resist is very effective for forming fine patterns and reducing a point defect such as a short in patterning of the ITO film, and therefore it is often used. A process for forming the transparent electrode is shown in FIG. 1. First, a polycrystalline ITO film 12 is formed on a substrate 10 by a conventional method such as a sputtering method (FIG. 1(a)). A transparent electrode is typically formed to have a thickness of approximately 400 to 1500 xc3x85. A negative chemically amplified resist 14 is provided directly on the polycrystalline ITO film 12 to have a thickness of approximately 1.5 to 2.0 xcexcm, and a part to be a pixel is irradiated (FIG. 1(b)). Then, the resist 14 is baked optionally and developed to obtain a resist pattern 16 (FIG. 1(c)). By using this pattern 16 as a mask, the ITO film 12 is etched in a etchant, for example, mixed aqueous solution of nitric acid and hydrochloric acid, and then the ITO film 12 can be patterned.
For the substrate after the resist development, some kinds of visual inspections, e.g. inspection of pattern defects, inspection of defects, inspection of contaminants or dust and so on, are performed. These inspections are essential to improvements of process yield and stability. However, it was found that if the substrate is placed under a white light or a green light for the inspection, adhesion of the resist pattern to the ITO film is reduced (FIG. 1(dxe2x80x2)), and in the ITO etching step which is subsequently performed, resist peeling or deterioration of line width distribution occurs (FIG. 1(exe2x80x2)). This problem may be attributed to the fact that photolysis reaction occurs in the photo-acid generating material contained in the resist, additional acid is generated in the resist pattern 16, and this acid corrodes the ITO film in the boundary between the ITO film 12 and the resist 16. In order to prevent the generation of additional acid, all the inspections mentioned above must be carried out under a yellow light which does not include an absorption spectrum of the photo-acid generating material, i.e. a light which wavelength of approximately 380 nm or lower should not be included (FIG. 1(d)). Under the yellow light, no resist peeling or no deterioration of the line width distribution occurs (FIG. 1(e)). In practice, however, it is very difficult to perform satisfactory inspection under the yellow light. Further, not only the inspection but also any steps including carrying before etching of the ITO film must be carried out under the yellow light while avoiding a white light. In order to do so, yellow lamps must be installed not only in an etching room but also all hinds of passages, AGV, stockers, and so on.
In order to solve the problem in which acid generated by exposing the chemically amplified resist diffuses into the electrically conductive substrate during the patterning by using the chemically amplified resist, Japanese Patent Laid-Open No. H6(1994)-132208 teaches us to provide an insulating thin film beforehand on the electrically conductive substrate, and provide the chemically amplified resist thereon. However, the provision of another layer between the layer to be patterned and the resist leads to increase in the number of steps and costs, as well as to reduce in yield.
An object of the present invention is to provide a process which solves a problem involved in resist peeling or reduced adhesion caused by light exposure after pattern formation, thereby achieving micro-patterning for an ITO film with a precision and accuracy.
Another object of the present invention is to provide a process for manufacturing a high image quality flat panel display with high yield by integrating, at a high density, a transparent electrode made of an ITO film. For this purpose, visual inspection of the patterned transparent electrode under a white light or a green light should be necessary.
According to the present invention, a novel process for patterning an ITO film is provided. The process comprises the steps of: preparing an amorphous ITO film on a substrate; applying a negative chemically amplified photosensitive material (or resist) directly on the amorphous ITO film, and exposing and developing the negative chemically amplified photosensitive material to form a pattern; and removing a part of the amorphous ITO film which is not covered with the pattern. According to the process of the present invention, even if the pattern of the chemically amplified photosensitive material is exposed to a white light or a green light for visual inspection, no resist peeling or no reduction in adhesion occurs.
Another advantage of the present invention is that in the step of removing the part of the amorphous ITO film, relatively mild acid such as oxalic acid or phosphoric acid can be used as etching solution instead of using such strong acid as hydrochloric acid or nitric acid. In this way, the possibility of damage by strong acid during etching, for example, corrosion of an underlayer made of metal such as aluminum, can be eliminated. Accordingly, a patterned ITO film can be easily formed even on a metal feature.
The patterned amorphous ITO film obtained by the process of the present invention is heated so as to be converted into a microcrystalline material, thereby obtaining a crystallized ITO film which exhibits chemical resistance and good electrical conductivity.
According to an aspect of the present invention, a novel process for manufacturing a TFT substrate used for a flat panel display is provided. A pixel electrode of this TFT substrate is formed by using the process for patterning a transparent electrically conductive film, which is provided by the present invention. Specifically, the process comprises the steps of preparing an amorphous ITO film on a substrate; providing a negative chemically amplified photosensitive material directly on the amorphous ITO film, and exposing and developing the negative chemically amplified photosensitive material to form a pattern; removing a part of the amorphous ITO film which is not covered with the pattern; removing the pattern; and heating the amorphous ITO film to be microcrystallized. With the process of the present invention, since visual inspection under a white light or a green light can be surely performed after the chemically amplified photosensitive material (resist) is developed to form a resist pattern, a TFT substrate used for a high image quality flat panel display can be manufactured with high yield.